4 replies to this topic

Dear all,

I am a Mechanical Engineering student from Alexandria University, Egypt.
I'm working on my final year project.
The project is a design of selected units of a third generation oil refinery.
My task is to completely design an atmospheric crude distillation column.

Please correct me if I’m wrong
First I need to know the assay of the crude that will be separated.
2nd Calculating the theoretical number of trays, feed tray and min. reflux ratio.
3rd Calculating the diameter of the column and tray spacing.

I have the crude assay, I know how to calculate the theoretical number of trays for a binary mixture (McCabe Thiele method) but not to a multi-component mixture.
Could any one tell me what to do or read?
Because i really don't know what to do.

Thanks for your concern,

Ahmed Ashraf

[Art Montemayor]

Ahmed:

Unless you already have some Chemical Engineering courses in Unit Operations under your belt, I don't believe you are going to even understand what is going on with respect to the calculation of a multi-component distillation process design.

This type of design is done by computer and not by hand-calculated methods.

If you are interested, you can read up on multi-component distillation in many text books that are now available. I studied under Dr. Charles D. Holland in 1960, when he was writing the pioneer book on multi-component distillation calculations via computer. His books may still be in print.

[Zauberberg]

Also refer to "Elements of petroleum processing" by D.S. Jones, and "Petroleum refinery engineering" by W. Nelson. These are excellent resources for studying multiple draw-off towers, such is the one in CDU.

[djack77494]

Interesting challenge. Designing a crude column is very unique and interesting (and you might say difficult). For a nice "clean" distillation column, you feed a mixture of two pure components into the middle of a trayed column and get a heavy rich product out the bottom, and a light ends rich product overhead. You have specifications for your overheads and bottoms products, a condenser on the top and a reboiler at the bottom. Direct, straightforward, clean.

A world apart is the Atmospheric Crude Distillation Column. You have no reboiler. Instead you apply heat to your entering crude stream usually using perhaps a dozen or more heat exchangers and then a fired heater. You usually desalt the feed stream enroute to the column. Frequently you "prefractionate" the crude stream; other times you have a simple flash operation before the column. Your feed is a mixture of so many zillions of compounds that even the most exacting of engineers and scientists have long ago thrown up their hands and admitted that it is impossible to rigorously analyze and define this mixture. In place of knowing what their crude is, they have allowed the concept of psuedo-component to be used to characterize the mixture. You have side drawoffs where product streams lighter than the bottoms, but not so light as the overheads can be withdrawn. But because they are withdrawn from the middle of the column, they contain excess lights and must be stripped. To generate these mid-column products and to balance heat loads, it is necessary to remove heat at various locations throughout the column using pumparound coolers. It's a very involved operation. But, I digress. Refer to the books previously mentioned. Though I'm not familiar with Holland's, I know that Nelson and Jones books are excellent references. You might also look over some of the trade journals such as "Oil & Gas Journal" and "Chemical Engineering" to mention just two. Actual design (and operation) of the hardware is another whole area of expertise. Look to books such as Kister's for help in that area. Good luck.

[Clayton]

You say that you are a mechanical Engineering Student.

Is it that you are required to design the mechanical aspects of the column?

Or is it just that they gave you assay data and said design a column based on this??

If its the former there are various standards you can find on the mechanical design of columns.

If it is the latter then along with those books mentioned before the "Handbook of Petroleum Processing by D.S.J. Jones" is a very useful book also. It outlines the design of an atmospheric crude column very well along with those mentioned before.

I believe that it is quite possible to do it by hand. Distillation columns were around before computers